306 RESUME. 
branches of the trees of upper regions, and was collected in very sparse numbers, 
while P. stolida, called compressa by Pilsbry, has been stated to occur in a single 
valley, where repeated search for it was futile. 
The six species under investigation differ greatly in their degrees of distinction, 
in their intrinsic diversification, and in the relative extent of their territories of 
occupation; accordingly each one presents features of peculiar and special interest 
in addition to those that they share in common. At the outset, a detailed descrip- 
tion is given of the geographical and topographical features of Tahiti, in order that 
the exact situations of the several colonies belonging to these species may be 
clearly defined and understood. ‘The tables give the numbers of each species that 
were found in one, few, or many of the sixty-two valleys where any snails whatsoever 
were obtained. 
In studying the animals for the purpose of qualitative and quantitative com- 
parison, the standard characters of the shells were employed, although the soft 
bodies have been preserved for any further investigation that may seem desirable. 
The well-established statistical methods that give the type measure and its variation 
in each of the characters defined quantitatively prove to be indispensable for the 
definition of precise degrees of similarity and difference in the case of comparable 
species, subspecies, and component groups of lesser order. 
CHAPTER III. 
The first species described, P. hyalina Broderip, is unique in the fact that it 
occurs on islands belonging to the Society, Cook, and Austral groups, but not in 
all of the members of any one group, even in the case of the last-named, which 1s 
regarded as its headquarters. ‘The shell is always clear white, and has thus reached 
the limit of specialization in the loss of color. The species is distributed widely in 
Tahiti where it has been regarded as invariable, but biometric methods reveal 
differences where none would appear without the employment of such means. 
Variations in the numerical abundance of this species in the fifty-one valleys 
of its collection are discussed at some length, with the conclusion that such varia- 
tions are to be regarded as the products of constitutional differences and not of 
external circumstances. It is true that the drier quadrants of Tahiti, and the 
larger, less humid valleys are most favorable for this type, but the members of the 
several hyalina colonies vary constitutionally, so as to possess diverse degrees of 
ability or inability to meet the particular conditions of their respective localities. 
The data of fecundity show the highest rate of any species. Using these for an 
analysis of post-embryonic elimination, it appears that about two-fifths of a new- 
born generation will survive to reproductive maturity. 
The writer’s studies on this species have been extended to the Cook Islands, 
where a few specimens were secured from Mangaia and Moki. The shells exhibit 
certain statistical peculiarities when they are compared with those from Tahiti. 
Passing to the general discussion of the unique distribution of P. hyalina, the 
fundamental facts are that it occurs in such widely separated islands as Tahiti of 
the Society Group, Moki and Mangaia of the Cook or Hervey Group, as well as in 
